1. Field of the Invention
The present invention relates to an apparatus and method for compensation of central wavelength shifting of a fiber grating by a substrate with Invar effect.
2. Description of the Related Art
In an optical fiber based communication system, there is a trend toward application of fiber grating. The fiber grating has a plurality of physical properties, such as sharp reflection angle, suited to application of multiplex wave dividing devices, de-multiplexers, laser source stabilization devices and chirped gratings.
It is required for the fiber grating to provide an output wavelength reliably and stably in a range of ambient temperature from −40° C. to 80° C. For example, for an uncompensated fiber grating in which a laser beam having a 1550-nanometer wavelength is applied, a central wavelength change of the fiber grating is 1 nanometer when the ambient temperature increases from −40° C. to 80° C. Since high quality multiplex wave dividing devices may have a sophisticated transmission wave channel of 0.8 nanometers, wave channel stabilization for the fiber grating is required to prevent wave channel variation, or errors can occur in the optical fiber transmission.
However, in most reliable conventional compensation mechanism, the central wavelength of the fiber grating is fixed. One conventional compensation mechanism introduces a ceramic substrate with negative thermal expansion coefficient, made of ZrV2O7, ZrW2O8, Sc2(WO4)3 or HfW2O8. Manufacture of the ceramic substrate of ZrW2O8 is described in U.S. Pat. No. 6,258,743 to Fleming et al. (2001), U.S. Pat. No. 5,322,559 to Sleight et al. (1994), U.S. Pat. No. 5,433,778 to Sleight et al. (1995), and U.S. Pat. No. 5,514,360 to Sleight et al. (1996). Manufacture of the ceramic substrate of LiAlSi2O6 is described in U.S. Pat. No. 6,066,585 to Swartz (2000). Materials of ZrW2O8 and ZrV2O7 is described in Alexander et al. (1996), and materials of ZrW2O8, Sc2(WO4)3 or HfW2O8 are described in Evans (1996, 1997).
Further, an application of the ceramic substrate with negative thermal expansion coefficient for compensation of the central wavelength of the fiber grating is disclosed in U.S. Pat. No. 6,081,641 to Chen (2000), which relates to a structure and method to compensate for temperature changes on wavelength shifting in DWDM. The DWDM is connected to substrates of different thermal expansion coefficients, such that the fused-fiber portion of the DWDM exhibits negative thermal expansion. As a result, temperature-induced wavelength shifts are minimized due to passive thermal compensation, which can be easily adjusted.
Another application of the ceramic substrate with negative thermal expansion coefficient for compensation of the central wavelength of the fiber grating is disclosed in U.S. Pat. No. 6,209,352 to Beall et al., which relates to an athernal optical device and a method for producing the device. In Beall et al., the athermal optical fiber reflective grating device has a negative expansion substrate of glass-ceramic, an optical fiber mounted on the substrate surface, and a grating defined in the optical fiber. However, the glass-ceramic applied in Beall et al. is not limiting.
In the above-mentioned prior arts, metalloid-based ceramic materials with negative thermal expansion coefficients are used as the substrate of the fiber grating to compensate for the central wavelength shifting. However, there are some metal-based materials with negative thermal expansion coefficients.
Generally, a heated body in solid state expands. In order to reduce the thermal expansion coefficients of the body in a predetermined range of temperature to zero or even a negative value, which refers to the “Invar effect”, a thermal shrink effect of the body is applied. The thermal shrink effect is generally a magnetic effect in solid state. Presently, the well-known iron/steel materials with negative thermal expansion coefficients include 35Ni-65Fe, 32Ni-64Fe-4Co, 37Fe-32Co-11Cr, and the noncrystalline alloy 83Fe-17B. The alloy with a zero or negative thermal expansion coefficient is an Invar alloy.